Modern mass digital data storage subsystems are used for storing data for use, for example, for processing by digital computer systems or the like. Typically such subsystems include one or more planar rotating disks onto which data, in the form of binary digits (bits) are written or from which data is read by means of one or more recording heads positioned proximate each disk surface. Each disk is formed from a substrate onto which a magnetic material has been deposited. During a writing operation, an electrical signal representative of the digital data bits to be written is applied to the recording head. The electrical signal enables the head to generate an alternating magnetic field that, in turn, forms patterns of magnetic domains of alternating polarity head in the magnetic material that passes under the head as the disk rotates. On the other hand, during a reading operation, the recording head is positioned proximate the areas of the disk on which the data bits were previously written. The recording head senses the variations in the magnetic field in the magnetic domains and generates an electrical signal in response, that is processed to recover and identify the data bits.
In modern disk drives, each recording head is mounted in a "slider," which is held by an arm. A spring force applied to the arm biases the slider toward the disk surface. A slider has a shape generally similar to an air foil, which cooperates with air entrained with the rotating disk, termed an "air bearing," to generate a lift force that overcomes the spring bias allowing the slider to essentially fly over the disk surface. The arm holding the slider is pivotable to position the slider at diverse radial positions over the disk surface, each radial position comprising a track in which the recording head can write data to or read data from. While the disk rotation is stopped, the slider typically rests on the disk surface in a region termed the "parking region." As the disk rotation accelerates during a start-up operation the slider will slide over the disk surface until the entrained air reaches a speed that provides sufficient lift to force to overcome the spring force biasing the slider toward the surface, and thereby enable the slider to lift off the disk surface. Similarly, as the disk rotation decelerates during a shut-down operation, the entrained air may slow sufficiently while the disk is still rotating that the slider will land and slide over the disk surface until the disk comes to a complete stop.
The reliability of mass storage subsystems, particularly of the disk drives therein, is of considerable importance in digital data processing systems and other systems in which they are incorporated. Failure of a disk drive can result in loss or corruption of data, which can have evident negative consequences. Disk drives can fail for a number of reasons, including failures in the disk drive mechanism preventing the disks from rotating properly. In addition, "head crashes" can occur in which a slider dives into a disk surface or contamination such as dust on the disk surface. Head crashes can damage the slider and recording head, and can also damage the magnetic material on a disk surface so that the previously-stored data would be lost. Other failure modes occur which are related to wear on the slider resulting from sliding over the disk surface prior to the lift-off during a start-up operation and after landing during a shut-down operation; excessive wear, due to failure of a slider to lift off in a timely basis during a start-up operation, or due to the slider landing too early during a shut-down operation, may cause generation of excessive thermal energy in the slider and recording head, which can cause undesirable wear and corrosion and early failure of the recording head.
Because failure of disk drives can have a deleterious effect on the operation of, for example, computer systems in which they are used and, in particular, the availability of data stored therein, it is desirable to be able to identify those drives which are likely to fail, at least before they are sold to customers and placed in service. In one test arrangement, the pattern of acoustic energy (that is, sound) that is generated by a disk drive during start-up or shut-down can give a clue as to whether the sliders in the drive are operating properly. In that arrangement, the amplitude of the acoustic energy that is generated by a particular slider will increase as the disk rotation accelerates until the slider lifts off the disk surface. When the slider lifts off, the acoustic energy will decrease rapidly. If the pattern of the amplitude of acoustic energy generated by a slider shows that the slider is, for example, unusually slow in lifting off, it may be determined that there is an increased likelihood that the slider is likely to fail and, accordingly, the slider may be replaced before the drive is placed into service. However, since the test arrangement will detect patterns of acoustic energy as generated by the entire disk drive, which will include acoustic energy contributions from all of the sliders, it may be difficult to identify which of the sliders in the disk drive should be replaced. Accordingly, while the acoustic test arrangement may be helpful in determining that one or more of the sliders should be replaced, it may not be sufficient to identify the particular sliders that should be replaced.